Scanning SQUID microscopes have been developed and used for acquiring spatially resolved images of physical properties of different objects by non-invasively measuring magnetic properties of materials and devices by means of superconducting quantum interference devices, also known as SQUID sensors. Prior magnetic imaging devices using SQUIDS have maintained spatial resolution on the scale of a millimeter or larger which is too large for microscopically resolving images needed in semiconductors/micro-electronics testing. Additionally, these devices also required placing samples in a vacuum. Some samples such as liquids or biological specimens cannot tolerate vacuum, thus it is not practical to measure sources of biomagnetism which are currently the focus of much of the existing low spatial resolution SQUID imaging.
U.S. Pat. No. 5,491,411 discloses methods and apparatus for imaging microscopic spatial variations in small currents and magnetic fields capable of providing measurements of magnetic fields with enhanced spatial resolution and magnetic field sensitivity. However, the device requires placing a sample within a dewar which may result in the unwanted destruction of the sample when it is exposed to cryogenic liquid or a vacuum. Arguendo, even if the sample is able to tolerate the vacuum environment or cryogenic medium, introducing the sample into the vacuum or cryogenic space for imaging is a somewhat cumbersome and time consuming task.
The problem was at least partially resolved by the apparatus for microscopic imaging of electrical and magnetic properties of a sample disclosed in U.S. Pat. No. 5,894,220. The device includes a housing having a first portion containing a cryogenic medium and a second portion enveloping a vacuum space. The cryogenic SQUID sensor is disposed within the vacuum space and in fluid communication with the cryogenic medium in the housing for heat exchange therewith. The sample for measurement is positioned outside of the housing, at room temperature or higher, and can be “seen” by the SQUID sensor through a thin window made in the wall of the housing. The output of the cryogenic SQUID sensor is monitored as it is scanned over the surface of the sample.
Another scanning SQUID microscope is described in the International Publication No. WO 00/20879. In this device, the SQUID sensor is scanned over the surface of the sample under study, particularly electronic circuit, and the measured data are subjected to spatial filtering and masking techniques in order to increase the spatial resolution and eliminate noise and edge artifacts in magnetic fields and electric field images of the sample.
In all scanning SQUID microscopes disclosed in the above-mentioned references the SQUID sensor loop is oriented to be in a plane parallel to the sample plane so that only the normal component Bz of the detected magnetic field is measured. As shown in FIG. 1, SQUID chip 10 secured to the lowermost point of a sapphire tip 12 (attached to a tube 18) is disposed in parallel with the plane of a sample 14. As the sample 14 moves in perpendicular directions X and Y. the SQUID sensor detects the magnetic field generated by the sample 14. Particularly, as shown in FIG. 2, a magnetic field B is generated by a current path 16, extending in this particular example along the axis Y. The SQUID chip 10 disposed distance Z0 from the current path 16, detects the normal component Bz of the magnetic field B. The problem associated with this technique results from the fact that each acquired data point is the magnetic field averaged over the area of the SQUID sensor projection on the direction of a scan. Since, as shown in FIG. 3, the whole area of the SQUID sensor 10 faces (downwardly) toward the sample, and the projection area of the SQUID sensor onto the sample plane is large, the spatial resolution is then limited to the size of the SQUID sensor projecting onto the sample plane.
The scanning SQUID microscope described in the International Publication No. WO 00/20879, slightly improves the spatial resolution by processing the obtained data through filtering and masking electronics. This technique however requires excessive processing hardware and software and includes the limitations associated with parallel orientation of the SQUID sensor to the plane of the sample.
It is known in the prior art to operate SQUID sensors in a negative feedback loop or flux-locked loop. Referring again to FIG. 1, in order to couple magnetic flux into the SQUID sensor for maintaining a flux-locked loop, or for applying the read-out flux required for other imaging schemes, a three-turn coil 20 is wrapped around the sapphire tip 12. In order to increase the mutual inductance between the SQUID sensor and the coil 20 it was suggested in U.S. Pat. No. 5,894,220 to fabricate the coil directly on the SQUID chip using photolithographic printing technique known in the art. This suggestion is, however, more difficult in practice, since it requires a larger area of the SQUID that causes limitations associated with the trade-off between the spatial resolution and the size of the SQUID chip as discussed in previous paragraphs.
Further, SQUID bias and readout wires 22 are coupled between the SQUID chip 10 and the processing equipment 24. It is clear to those skilled in the art, that, as shown in FIG. 1, the contact between the wires 22 and the SQUID chip 10 is difficult to fabricate. Additionally, due to limitations applied to the size of the SQUID chip, the contact resistance to the device can be undesirably high if the contact pages are made too small.
It is therefore clear, that a different approach to the scanning SQUID microscope technique would be desirable to increase spatial resolution thereof without the necessity of using a rather complex processing technique as was proposed in the prior art. The subject system is directed to removing the limitations associated with the size of the SQUID and to afford a larger size of SQUID chip for accommodating modulation and feedback lines, as well as enlarged contact pads positioned thereon.